ABSTRACT Alzheimer's disease (AD) is the most common dementia in the elderly population and one of the leading causes of death in the developed world. Compelling evidences point that the hallmark event in AD is the misfolding, oligomerization and brain accumulation of protein aggregates in the form extracellular amyloid beta (A?) plaques and intracellular neurofibrillary tangles (NFTs). The pathological accumulation of these proteins in the brain has been attributed to an imbalance between the production and removal of misfolded aggregates. Clearance mechanisms include proteolytic degradation by several enzymes, phagocytosis by glial cells, transvascular transport across the blood brain barrier and interstitial fluid (ISF) flow into the cerebrospinal fluid (CSF) and clearance into the peripheral lymphatic system. ISF and CSF are exchanged via the glymphatics that drain into the peripheral lymphatic vasculature within the head and neck area. CSF clearance also takes place through the cribriform lymphatics to the cervical lymph nodes as well as through the meningeal lymphatic vessels. An emerging concept in the field is that lymphatic system impairment may contribute to AD pathology. Our preliminary results show that animals harboring amyloid plaques have a significantly impaired CSF outflow into cervical lymph nodes compared with animals without A? deposits. Moreover, our results, as well as various published observations, have shown a substantial impairment of the lymphatic circulation during aging. The main goal of this project is to determine the role of age-related alterations in brain fluid flow through the lymphatic system in the clearance of misfolded protein aggregates and the pathogenesis of AD. Our overarching hypothesis is that in aged people, impairments in CSF/ISF exchange and decrease in peripheral lymphatic function leads to a reduction on the clearance of A? and Tau misfolded aggregates, resulting in the net accumulation of these structures in the brain. Progressive deposition of cerebral protein aggregates further obstruct the fluid flow in the brain leading to an exacerbation of lymphatic dysfunction, which in turn further limits the clearance of these toxic species from the brain, resulting in a vicious cycle of pathogenesis. To test this hypothesis we will perform non-invasive near-infrared fluorescence imaging of lymphatic flow in animals of different ages as well as in various transgenic mice models that accumulate amyloid (either parenchymal plaques and cerebral amyloid angiopathy) or NFTs. We will also measure the brain clearance and net accumulation of A? and Tau aggregates in vivo at distinct ages as well as in animals with pharmacologically or surgically altered lymphatic function. Finally, we will study in humans whether the peripheral lymphatic function as well as the drainage into cervical lymph nodes is compromised in patients affected by AD compared to age-matched controls. We will correlate the lymphatic alterations with the disease severity, measured by cognitive and neuropsychological testing as well as by the use of established biomarkers. Finally, we will compare the lymphatic changes in AD patients with the presence and concentrations of A? and Tau misfolded oligomers in CSF measured by the protein misfolding cyclic amplification. Our project combines the resources, expertise and facilities available at 3 different groups: Soto (AD, animal models, brain clearance studies), Sevick (animal and human imaging of lymphatic function) and Schulz (clinical studies with AD patients) to study in a comprehensive manner the putative role of the glymphatic/lymphatic system in AD. The results obtained in this study may contribute to understand the role of the brain fluid outflow and the peripheral lymphatic system in AD, which may translate in new targets for therapeutic intervention.